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AN4235Application note

I2C timing configuration tool for STM32F3xxxx and STM32F0xxxx microcontrollers

IntroductionThis application note presents the I2C timing configuration tool (STSW-STM32126) for the STM32F3xxxx and STM32F0xxxx microcontroller families.

The STM32F0xxxx and STM32F3xxxx devices embed an Inter-Integrated Circuit communication peripheral (I2C) supporting standard mode (100 KHz), fast mode (400 KHz) and fast mode plus (1 MHz). The I2C implements a new clock scheme allowing the peripheral to be used as a wake-up source from low-power mode on address match.

The purpose of this tool is to help the user configure the I2C timings, taking into consideration the I2C bus specification.

The configuration tool is implemented in the Microsoft Excel “I2C_Timing_Config_Tool_Vx.y.z.xls” file which can be downloaded from www.st.com.

For Vx.y.z, please refer to the tool version, for example: V1.0.0.

Before using the clock tool, it is essential to read the STM32 microcontroller reference manuals (RM0313 for STM32F37xxx products, RM0316 for STM32F30xxx products and RM0091 for STM32F0xxxx products). This application note is not a substitute for the reference manuals.

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Contents

1 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1 Software requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Hardware requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 I2C timing specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.3 I2C clock scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.4 I2C timing register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Tutorials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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List of tables

Table 1. Definition of terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Table 2. I2C timings specification (see I2C specification, rev.03, June 2007) . . . . . . . . . . . . . . . . . . 8Table 3. Timing register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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List of figures

Figure 1. I2C bus timing (see I2C specification, rev.03, June 2007) . . . . . . . . . . . . . . . . . . . . . . . . . . 7Figure 2. I2C clock scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Figure 3. Data setup time generation from SCLDEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 4. Data hold time generation from SDADEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Figure 5. High and low period generation from SCLH and SCLL . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Figure 6. I2C timing configuration tool user interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Figure 7. Calculation is completed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 8. Error message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 9. Warning message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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1 Glossary

Table 1. Definition of terms Term Description

AF Analog filter

DNF Digital noise filter

HSI High-speed internal clock

I2C Inter-Integrated Circuit

I2CCLK I2C kernel clock

PCLK APBx clock

PRESC Prescaler

SCL Serial clock line

SDA Serial data line

SYSCLK System clock

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2 Getting started

This section describes the requirements and procedures needed to start using the timing configuration tool.

2.1 Software requirementsTo use the timing configuration tool with Windows operating system, a recent version of Windows, such as Windows XP, Vista or Windows 7, must be installed on the PC with at least 256 Mbytes of RAM.

Before starting to use the timing configuration tool, make sure that Microsoft Office is installed on your machine and then follow these steps:• Download the latest version of the I2C timing configuration tool for the STM32

devices from www.st.com.• Enable macros and ActiveX controls as shown below:

Excel 1997-2003 version

1. Click Tools in the menu bar.2. Click Macro.3. Click Security.4. Click Low (not recommended).

Note: If ActiveX controls are not enabled, a warning message is displayed asking you to enable ActiveX. In this case, you should click “OK” to enable it.

Excel 2007-2010 version

1. Click the Microsoft Office button and then click Excel options.2. Click Trust Center, click Trust center settings, and then click Macro settings.3. Click Enable all macros (not recommended, potentially dangerous code can run).4. Click Trust Center, click Trust center settings, and then click ActiveX settings.5. Click Enable all controls without restrictions and without prompting (not

recommended; potentiality dangerous controls can run).6. Click OK.

Note: For more information about how to enable macros and ActiveX controls, refer to the Microsoft Office website.

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2.2 Hardware requirements

2.2.1 IntroductionThe I2C timing configuration tool is designed to help the end-user easily configure the timing settings for the I2C peripheral and guarantee its operation as specified in the I2C timing specification.

2.2.2 I2C timing specificationThe I2C timings should be configured with values that are compliant with the I2C bus specification:

Figure 1. I2C bus timing (see I2C specification, rev.03, June 2007)

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The table below shows the value range of these timings:

Table 2. I2C timings specification (see I2C specification, rev.03, June 2007)

Symbol ParameterStandard Fast mode Fast mode +

UnitMin Max Min Max Min Max

fSCL SCL clock frequency 0 100 0 400 0 1000 KHz

tLOW Low period of the SCL clock 4.7 - 1.3 - 0.5 - µs

tHIGH High Period of the SCL clock 4 0.6 0.26 - µs

trRise time of both SDA and SCL signals - 1000

20 + 0.1Cb(1) 300 - 120 ns

tfFall time of both SDA and SCL signals - 300

20 + 0.1Cb(1) 300 - 120 ns

tHD;DAT Data hold time 0 - 0 - 0 - µs

tVD;DAT Data valid time - 3.45(2) - 0.9(2) - 0.45(2) µs

tVD;ACK Data valid acknowledge time - 3.45(2) - 0.9(2) - 0.45(2) µs

tSU;DAT Data setup time 250 - 100 - 50 - ns

tHD:STA Hold time (repeated) START condition 4.0 - 0.6 - 0.26 - µs

tSU:STA Set-up time for a repeated START condition

4.7 - 0.6 - 0.26 µs

tSU:STO Set-up time for STOP condition 4.0 - 0.6 - 0.26 - µs

tBUFBus free time between a STOP and START condition 4.7 - 1.3 - 0.5 - µs

1. Cb = total capacitance of one bus line in pF.

2. The maximum tHD;DAT could be 3.45 µs, 0.9 µs and 0.45 µs for standard mode, fast mode and fast mode plus, but must be less than the maximum of tVD;DAT or tVD;ACK by a transition time. This maximum must only be met if the device does not stretch the LOW period (tLOW) of the SCL signal. If the clock stretches the SCL, the data must be valid by the set-up time before it releases the clock.

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2.2.3 I2C clock schemeThe I2C kernel is clocked by an independent clock source. The clock source can be:• HSI (default source)• SYSCLK

Figure 2. I2C clock scheme

These two clocks allow I2C to operate independently from the PCLK frequency.

Setting HSI as I2C clock source frequency allows the use of wake-up from STOP mode capability at address match.

The I2CCLK period tI2CCLK must respect the following conditions:

tI2CCLK < (tLOW - tfilters) / 4 and tI2CCLK < tHIGH

tfilters: when enabled, sum of the delays brought by the analog filter and the digital filter.

Analog filter delay is maximum 260 ns and digital filter delay is DNF x tI2CCLK.

The PCLK clock period tPCLK must respect the following condition:

tPCLK < 4/3 tSCL

Please refer to the RCC section in STM32 product reference manual for more details about the selection of the I2C clock source.

I2C Interface

Timing generation

Registers

I2CCLK

RCC I2CxSWPCLK

SYSCLK

HSI

MS31629V1

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2.2.4 I2C timing registerThe I2C timing register is defined as the following table shows:

Table 3. Timing register

PRESC[3:0] is used to prescale I2C clock source (I2CCLK); it allows the generation of a divided clock. The period of this divided clock tPRESCis defined by:

tPRESC = (PRESC+1) x tI2CCLK

The time unit tPRESC is used for the generation of other I2C timings.

SCLDEL[3:0] is used to program the data setup time (tSU;DAT) as shown in the following figure:

Figure 3. Data setup time generation from SCLDEL

SCLDEL is defined as follows:

{[tr+ tSU;DAT(min)] / [tPRESC]} - 1 <= SCLDEL

SDADEL[3:0] is used to program the data hold time (tHD;DAT) as shown in the following figure:

SCLDEL

SCL

SDA

Data setup time MS31640V1

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Figure 4. Data hold time generation from SDADEL

tSYNC1 duration depends on these parameters:• SCL falling time• When enabled, input delay brought by the analog filter: 0.05 µs < tAF < 0.26 µs• When enabled, input delay brought by the digital filter: tDNF = DNF x tI2CCLK• Delay due to SCL synchronization to I2CCLK clock (2 to 3 I2CCLK periods)

SDADEL is defined by:

SDADEL >= {tf +tHD;DAT(min) - tAF(min) - tDNF - [3 x tI2CCLK]} / {tPRESC}

SDADEL <= {tVD;DAT(max) - tr - tAF(max) - tDNF- [4 x tI2CCLK]} / {tPRESC}

SCLH[7:0] and SCLL[7:0] are used to configure I2C speed frequency when master mode is selected. SCLH generates the high period of the SCL clock (tHIGH) and SCLL generates the low period of the SCL clock (tLOW). The figure below shows how these timings are deduced:

Figure 5. High and low period generation from SCLH and SCLL

tSYNC2 duration depends on these parameters:• SCL rising time• When enabled, input delay brought by the analog filter: 0.05 µs < tAF < 0.26 µs• When enabled, input delay brought by the digital filter: tDNF = DNF x tI2CCLK• Delay due to SCL synchronization to I2CCLK clock (2 to 3 I2CCLK periods)

tSYNC1 SDADEL

SCL

SDA

Data hold timeMS31639V1

tSYNC1

SCLH

SCL

SCLLtSYNC2

SCL high level

SCL low level

MS31641V1

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SCL clock period (tSCL) which defines I2C speed frequency (fSCL= 1/tSCL) is defined by:

tSCL = tf + tLOW + tr + tHIGH

SCLH and SCLL are defined as follows:

tHIGH(min) <= tAF(min) + tDNF + 2 x tI2CCLK + [(SCLH+1) x tPRESC]

tLOW(min) <= tAF(min) + tDNF + 2 x tI2CCLK + [(SCLL+1) x tPRESC ]

Note: SCLH and SCLL values depend on the rise and fall time. The rise time is defined by:tr = Rp x Cb x 0.8473 (Rp is the pull_up resistor and Cb is the bus capacitance)The fall time depends on the software configuration of the I/O. Please refer to “I/O AC characteristics” table in STM32 products datasheets to get the value of fall time.

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3 Tutorials

This section describes how to use the I2C timing configuration tool.

Figure 6. I2C timing configuration tool user interface

Note: The “Reset” button resets the input parameters to their default configuration.To get the value of the timing register, follow these steps:1. Select device mode by choosing “Master” or “Slave” in the list box.2. Configure the speed mode by selecting one of the following modes in the list box:

– Standard mode: maximum frequency is 100 KHz.– Fast mode: maximum frequency is 400 KHz.– Fast mode Plus: maximum frequency is 1000 KHz.

3. Set the desired I2C speed frequency (master clock).4. Set the value of I2C clock source frequency.5. Specify if analog noise filter is enabled or not.6. Specify if digital noise filter is used or not by setting the filter coefficient (this coefficient

should be an integer from 0 to 15).7. Set the value of rise time.8. Set the value of fall time.

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9. Click the RUN button:a) If the calculation of the timing register is completed, the following message is

displayed:

Figure 7. Calculation is completed

In this case, you can copy the generated value from TIMINGR register value test-box and use it to configure the I2C timing (a double click in the result box copies the value).

Here is an example showing how to use the generated value to initialize the I2C timing register using the standard peripheral library of STM32 products.

We suppose that:• Master mode is selected and desired I2C speed frequency is 100 KHz in fast mode.• The I2C clock source frequency is 48 MHz with SYSCLK as source.• Analog and digital noise filters are disabled.• Rise time value is 65 ns and the fall time value is 5 ns.

The generated value for this configuration is 0x0070D8FF.

Follow this steps to configure the I2C peripheral:1. Declare the I2C initialization structure:

I2C_InitTypeDef I2C_InitStructure;2. Initialize structure parameters:

I2C_InitStructure.I2C_Mode = I2C_Mode_I2C;I2C_InitStructure.I2C_AnalogFilter = I2C_AnalogFilter_Disable;I2C_InitStructure.I2C_DigitalFilter = 0x00;I2C_InitStructure.I2C_OwnAddress1 = 0x00;I2C_InitStructure.I2C_Ack = I2C_Ack_Enable;I2C_InitStructure.I2C_AcknowledgedAddress = I2C_AcknowledgedAddress_7bit;I2C_InitStructure.I2C_Timing = 0x0070D8FF;

3. Call I2C_Init() function:I2C_Init(I2C1, &I2C_InitStructure);

b) If the user configuration does not provide a result compliant with the I2C timing specification, this message is displayed:

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Figure 8. Error message

In this case, you should try to generate the timing register value with a different configuration.

Note: In case that the maximum hold time value violates the I2C timing specification, a warning message is displayed and the I2C timing is calculated:

Figure 9. Warning message

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4 Conclusion

This application note provides a brief description of the I2C timing register configuration and explains how to use the I2C timing configuration tool with the STM32 microcontroller devices.

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5 Revision history

Table 4. Document revision historyDate Revision Changes

24-Jan-2013 1 Initial release

07-Aug-2013 2

Document reformatted. Updated:– Introduction.– Table 1: Definition of terms.– Note 2 in Table 2: I2C timings specification (see I2C

specification, rev.03, June 2007).– Section 2.2.4: I2C timing register.– Section 3: Tutorials:

- Replaced "0xA0120227" by "0x0070D8FF" - Added Figure 9: Warning message and related note above.

Improved Figure 3, Figure 4 and Figure 5.

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